Cleaning and preparing root canals of a tooth for receiving filling material is accomplished using drilling instruments with an active portion called the working portion, the purpose of which is to shape and clean the root canal in preparation for receiving the materials used to treat and fill it.
Root canals often have specific shapes with complex curves and narrow cross-sections formed of constricted or oval areas that do not lend themselves to the introduction of preliminary shaping instruments. This is why instruments known as files must have characteristics that sometimes are contradictory: the files must be fine but resistant, yet flexible enough to conform to the curves of the root canal and reach the end of the canal, while nevertheless remaining durable enough to shape and cut the walls of the canal.
These exigencies oblige the odontologist to undertake a preparatory root canal treatment process using a broad array of tools and working progressively to adapt to the root canal morphology, the array of tools having various structures and dimensions. The intervention begins with a flexible fine instrument which will then be replaced by instruments of increasing cross-section until the root canal has an interior cavity large enough to receive the filling material. This is a long, delicate series of operations, being mindful that for safety reasons, the treating and filling material must completely fill the root canal and taking precautions to ensure that no residual air remains at the base of the cavity created in order to prevent any growth of bacteria and eventual infection.
These instruments are difficult to introduce into the root canal. In addition, to date there is no universal instrument adapted to the morphology of the root canal to be treated which would perform all the preparatory operations in one procedure. There is a risk of instruments cracking, becoming blocked in the canal, or greatly over-heating, which may cause breakage. This risk is notoriously present when using mechanically driven instruments made of nickel-titanium alloy, which wear out and must be carefully monitored by the odontologist throughout use. There is no doubt that using several different instruments in succession increases not only the cost of the intervention, but the complexity of the odontologist's work and risk to the patient.
U.S. Publication No. US2010/0233648 describes a method and an endodontic instrument made of superelastic material. A rod of superelastic material is set into a shaped configuration to form an instrument, such that the instrument may be inserted into a root canal in a configuration different than the shaped configuration and revert towards its shaped configuration during the endodontic procedure. In order to form the rod into a desired instrument shape, the rod is compressed (preferably ranging from about 550 MPa to about 1500 MPa) between heated forming surfaces (typically between about 100° C. to about 200° C.) which cause a stress on the rod.
When the instrument is made of a flexible metal alloy, the instrument is designed to resume its retracted shape through a mechanical action after it has been used in the expanded structured shape.
A significant drawback to conventional root canal procedures is that a practitioner must generally use a series of endodontic files to clean out and shape a diseased root canal. Typically, this series of instruments consists of a set of files of increasingly larger diameter and, as a result, an increasing taper, as the length of the working portion is often maintained substantially constant. Sets of such files are used to sequentially and gradually enlarge the root canal until the desired shape is achieved. A stepped enlargement in relatively small increments is believed to be an important part of the conventional strategy of avoiding undesirable damage to and other effects on the tooth structure during the enlargement process, and in avoiding imposition of too much torsional load or stress on the material comprising the instrument. In this regard, a set of instruments are often used only once for a particular patient and then discarded, with each instrument in the set provided at substantial individual expense. Accordingly, there is a need for an improved endodontic file design that limits the number of endodontic files necessary to achieve a desired bore shape or degree of enlargement during root canal therapy/filing procedures.